BUG: define set with `set([...])` for python 2.6
BUG: define set with `set([...])` for python 2.6

compatibility, to avoid breaking older code
without significant reason. This change is not
motivated by any other reason.

Support for python < 2.7 will **not** be
actively maintained.

erocarrera/pydot,pydot/pydot